I have recently bought an Archer T3U(EU) v1.0, which, according to the specs and packaging is USB3.0 capable. However, I can only get the device recognized as a USB2.1 device, no matter where I try to use it.

I have tried plugging it into a little single-board computer (Odroid N2), my MacBook Pro, and a Windows/Linux desktop computer, all equipped with functioning USB 3 ports.

See some diagnostics from all of these machines below (relevant parts in bold). The Archer T3U (2357:012d) is in every single case only recognized as a USB 2.1 device.

And indeed, I only get the transfer speeds I'd expect from the USB 2.1 standard (~30MB/s with iperf3).

For your information, this T3U supports self-adaptive function, that is to say, as long as your device connected to the 2.4GHz, the adapter will be recognized as an USB2.0 device, while once it is connected to the 5GHz, it will be recognized as an USB 3.0 device.

For example, with your Mac connected to the 2.4GHz, when you check the properties of the adapters, it shows up as an USB 2.0 device, while if you switched to 5GHz and re-open the properties page, it will be an USB 3.0 device.

Unfortunately it does not help. I was already connecting to the 5GHz band in my above experiments, and I have additionally now disabled 2.4GHz functionality in my router. But to no avail - my T3U keeps getting recognized as USB 2.1 only, on all my systems.